1. Field of the Invention
The present invention relates to a nitride single crystal substrate, and more particularly, to a nitride single crystal substrate capable of facilitating a manufacturing process of a light emitting device and a vertical nitride light emitting device using the same.
2. Description of the Related Art
In general, a group III nitride semiconductor emits a wide range of light from a visible light to an ultraviolet ray. The group III nitride semiconductor has been greatly highlighted as an optical device material for realizing bluish green light.
Typically, a nitride single crystal has been produced on a heterojunction substrate by a vapor growth method such as Metal Organic Vapor Phase Epitaxy (MOVPE), Hydride Vapor Phase Epitaxy (HVPE) or a Molecular Beam Epitaxy (MBE) method. Examples of the heterogeneous substrate include a sapphire (α-Al2O3) substrate or a SiC substrate. But the sapphire substrate has lattice constant mismatch of about 13% with nitride gallium. Also, the sapphire substrate has a considerable difference (−34%) in thermal expansion coefficient with nitride gallium. This causes stress to an interface between the sapphire substrate and a nitride gallium single crystal, thereby resulting in lattice defects and cracks in the single crystal.
In a recent attempt to overcome this problem, the nitride semiconductor device has been directly grown on a homojunction substrate, i.e., a nitride single crystal substrate. As a prerequisite therefor, a technology for fabricating a free-standing nitride single crystal has been vigorously studied.
To produce the free standing nitride single crystal substrate, as shown in FIG. 1a, first, a nitride single crystal bulk 12 is grown on a base substrate 11 such as a sapphire substrate. Then, as shown in FIG. 1b, the base substrate 11 is removed from the nitride single crystal bulk 12. Here, the sapphire substrate is removed by laser lift-off.
The nitride single crystal substrate may be conductive contrary to the sapphire substrate, which thus allows easy manufacture of a vertical light emitting device superior in current spreading efficiency. However, in reality, even with use of the nitride single crystal substrate, there is a difficulty in fabricating the vertical light emitting device.
More specifically, the nitride single crystal substrate needs to be highly doped to be sufficiently conductive. Otherwise, it entails a complicated process since the nitride single crystal substrate should be entirely removed by mechanical polishing to fabricate a vertical light emitting device.
Alternatively, the nitride single crystal substrate may be highly doped with specific impurities to be sufficiently conductive. Yet, higher concentration of impurities in the nitride single crystal increases stress in the single crystal and accordingly defect density, thereby leading to degradation of crystal quality such as decline in carrier mobility. Such doping with specific impurities may inflict cracks on a large-scale substrate which is manufactured.